Multi-path interference filter with reflective surfaces

ABSTRACT

An interference filter (10, 30, 50, 70, 90, 110, 130, 150, or 190) filters selected wavelengths by dividing an input beam into two or more intermediate beams having different optical path lengths and by recombining the intermediate beams into an output beam that is modified by interference between the intermediate beams. An optical path length difference generator (20, 40, 60, 80, 100, 120, 140, 160, or 200) varies the optical path lengths of the intermediate beams by changing the physical lengths of their paths or the refractive indices of the mediums in which they are conveyed. The optical path length generator (20) of one exemplary embodiment (10) includes a spacer plate (20) that is divided into elements (22 and 24) having different refractive indices for varying the optical path lengths of the intermediate beams. Another optical path length difference generator (140) is formed by a stack of partially reflective surfaces (144) that are spaced apart in the direction of beam propagation by at least one nominal wavelength for varying the physical path lengths between the intermediate beams.

RELATED APPLICATIONS

This application is a Continuation-In-Part of copending allowed parentApplication Ser. No. 08/784,020, filed Jan. 15, 1997 now U.S. Pat. No.5,841,583, by Venkata A. Bhagavatula, entitled MULTI-PATH INTERFERENCEFILTER, which parent application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/011,444, filed on Feb. 9, 1996. Both parent andprovisional applications are hereby incorporated by reference.

TECHNICAL FIELD

The invention relates to optical devices for filtering selectedwavelengths of light using the mechanism of interference.

BACKGROUND

Interference filters use combinations of constructive and destructiveinterference to shape filter responses. Wavelengths subject toconstructive interference pass through the filters, and wavelengthssubject to destructive interference are blocked. The interference iscreated by overlapping different phase-shifted portions of the samebeam. Examples include Fabry-Perot etalons, dielectric filters, andfiber Bragg gratings.

Fabry-Perot etalons use pairs of opposing partially reflective surfacesto produce multiple interference between reflected beam portions.However, the filter response is limited. Sinusoidal response curves aretypical. Manufacturing is complicated by a requirement for precisealignment of the reflective surfaces.

Both dielectric filters and fiber Bragg gratings have alternating layersof high and low refractive index to produce a series of partialreflections that are offset by the spacing between the layers.Typically, the layers are spaced apart by one-quarter of the nominalwavelength of the filtered beam, which is difficult to hold for assemblyof dielectric filters. Conventional manufacturing of the dielectricfilters is limited to bulk optics, which are generally more costly thancomparable integrated optics.

The index variation of fiber Bragg gratings is very low (e.g., 0.0001)so a very large number of layers are required to attenuate unwantedwavelengths. The alternating layers are made by exposing aphotosensitive material to a standing wave. This limits the choice ofmaterials to those which are photosensitive.

U.S. Pat. No. 4,715,027 to Mahapatra et al. discloses amulti/demultiplexer that can also be arranged as a filter. An echelongrating has reflective surfaces arranged in a staircase to reflect lightback to a source at equally spaced frequencies. Although the filter canbe manufactured as an integrated optic, its response is also limited.The filters must be cascaded in succession similar to a vernier tofurther refine the response.

SUMMARY OF INVENTION

My invention also filters selected wavelengths of light by dividing ainput beam of light into two or more intermediate beams having differentoptical path lengths and by recombining the intermediate beams into anoutput beam that is modified from the input beam by interference betweenthe intermediate beams. The optical path lengths of the intermediatebeams can be varied by altering either the physical lengths of theirrespective paths or the refractive indices of the mediums in which theyare conveyed.

My multi-path filter includes input and output optical pathways and anoptical path length difference generator. One or more focusing opticscan be used to couple the optical path length difference generator tothe input and output pathways. For example, the input and output opticalpathways can be aligned with an optical axis together with a pair offocusing optics and a spacer plate that functions as the optical pathlength difference generator. The first focusing optic collimates anexpanding input beam emitted from the input pathway. The spacer platehas two or more intermediate pathways that divide the collimated beaminto a corresponding number of intermediate beams having differentoptical path lengths. The second focusing optic converges and recombinesthe intermediate beams at a focus on the output optical pathway.

Another configuration of my multi-path filter includes adjacent inputand output optical pathways that are offset from the optical axis. Asingle focusing optic cooperates with a spacer plate and a reflectiveoptic for both collimating an input beam from the input pathway andfocusing returning intermediate beams on the output pathway. With theaddition of the reflective optic, the intermediate beams traverse thespacer plate two times, thereby doubling the differences between theiroptical path lengths. Alternatively, the reflective optic can be curvedto also perform the functions of the focusing optic. A diverging beamfrom the input pathway is reflected on a return course through theintermediate pathways as a converging beam to the output pathway.

The intermediate pathways of the spacer plate can be varied in number,material, transverse area, and longitudinal length for achieving adesired spectral response. Optical path length differences betweenintermediate beams are created by varying the refractive indices of thematerials forming the intermediate pathways, by varying their length, orboth. The transverse areas of the intermediate pathways control therelative energies of the intermediate beams, and the number of differentintermediate pathways controls the number of intermediate beams that cancontribute to the desired spectral response. In general, the number ofintermediate pathways is analogous to the number of slits in aconventional interference model.

In place of the spacer plate, a reflective stack can be used to generateoptical path length differences between the intermediate beams. Eachlayer of the reflective stack has a reflective coating that reflects aportion of the total light that is incident to the stack. The partialreflections provide a plurality of intermediate pathways that overlapspatially but separate the intermediate beams through different physicalpath lengths of at least one wavelength. The number, material, partialreflectivity, and longitudinal length of each layer can be controlled toadjust the spectral response.

My filter can be implemented in bulk optics, integrated optics, or invarious hybrid combinations. For example, all of the elements can beconstructed in planar technology. However, separately oriented elementsare preferably used for more accurately reflecting light parallel to theother elements. My filter can also be incorporated within a singlefiber. Two focusing optics and a spacer functioning as an optical pathlength difference generator are fusion spliced to join two ends of thefiber.

Materials within the spacer of the fiber or other implementation can becombined to exhibit different index characteristics with externallycontrolled conditions such as temperature, pressure, or electrical ormagnetic fields. The controlled variation in the index differencebetween different intermediate pathways can be used for tuning thefilter response to selected wavelengths.

DRAWINGS

FIG. 1 is an optical diagram of my multi-path interference filter usinga two-part spacer block for generating optical path length differencesbetween axially aligned input and output fibers.

FIG. 2 is a graph of an exemplary spectral response of my filter as ameasure of transmitted intensity over a domain of wavelengths.

FIG. 3 is an optical diagram of a similar interference filter with analternative spacer block.

FIG. 4 is an optical diagram of a similar multi-path interference filterwith an alternative spacer block having two concentric elements.

FIG. 5 is an optical diagram of my multi-path interference filter withadjacent input and output fibers optically connected by an alternativespacer block and a reflective optic.

FIG. 6 is an optical diagram of a similar interference filter in whichan alternative spacer block includes additional elements for providing amore complex spectral response.

FIG. 7 is a graph similar to the graph of FIG. 2 but showing a morecomplex spectral response provided by the additional spacer blockelements.

FIG. 8 is an optical diagram of a similar interference filter with analternative spacer block for providing a complex spectral response.

FIG. 9 is an optical diagram of my multi-path interference filter inwhich a reflective stack functions as an optical path length differencegenerator.

FIG. 10 is a cross-sectional view of an optical fiber incorporating mymulti-path interference filter formed by two GRIN lenses and a spacerthat generates optical path length differences.

FIG. 11 is a sectional view along line 11--11 through the spacer showingtwo different optical segments for providing the different length paths.

FIG. 12 is a similar sectional view of an alternative spacer enclosed bytwo electrodes for tuning the filter's spectral response.

FIG. 13 is a cross-sectional view of a single mode fiber fused to amulti-mode fiber with a Gaussian index profile.

FIG. 14 is a cross-sectional view of the same two fibers with themulti-mode fiber cleaved to a length appropriate for use as a GRIN fiberrod lens.

FIG. 15 is an optical diagram of an interference filter similar to thefilter of FIGS. 4-6 with a reflective surface shaped to take on thefunctions of a focusing optic.

DETAILED DESCRIPTION

One embodiment of my invention as a multi-path filter 10 implemented inbulk optics is illustrated by FIG. 1. The illustrated filter 10 hassingle-mode input and output fibers 12 and 14 (input and outputpathways) optically coupled by two lenses 16 and 18 (focusing optics)and a spacer plate 20 that functions as an optical path lengthdifference generator. The first lens 16 changes an expanding input beamfrom the input fiber 12 into a collimated beam that strikes the spacerplate 20 at normal incidence.

The spacer plate 20 is divided into two different optical elements 22and 24 (intermediate pathways) that extend parallel to differenttransverse sections of the collimated beam. The element 22 is formed bya material having a first refractive index "n₁,", and the element 24 isformed by a material having a second refractive index "n₂ ". The twoelements 22 and 24 of the spacer plate 20 divide the collimated beaminto parallel intermediate beams having different optical path lengthsas determined by the following equation:

    ΔOPL=L (n.sub.1 -n.sub.2)                            (1)

where "ΔOPL" is the optical path length difference of the twointermediate beams and "L" is the longitudinal length of the spacerplate in the direction of propagation.

The second lens 18 converges and recombines the two intermediate beamsto a focus at the inner end of the output fiber 14, which is locatedalong a common optical axis 26 with the input fiber 12. When recombined,constructive interference occurs periodically at wavelengths "λ" thatare integer multiples "M" of the path length difference "ΔOPL".Expressed mathematically:

    ΔOPL=Mλ                                       (2)

A typical spectral response curve for the filter 10 is shown in FIG. 2.The original spectral power distribution of the beam entering the filter10 is assumed to be even throughout the domain of measured wavelengths.The response curve 28, which reflects the spectral power distribution ofthe beam upon exiting the filter 10, has a cyclical form with peakintensities located at wavelengths subject to maximum constructiveinterference between the beams, i.e., the wavelengths that are integermultiples of the path length difference. All other wavelengths aresubject to varying degrees of destructive interference. A preferredrange for the multiple "M" is between 20 and 150.

Another multi-pass filter 30, which is shown in FIG. 3, includes similarinput and output fibers 32 and 34 and lenses 36 and 38 but a differentspacer plate 40. The spacer plate 40 is made from a base element 42 andan extension element 44 that distinguishes the spacer plate 40 in thetransverse direction. Assuming that the base element 42 is homogeneous,the only dimension of interest to the optical path length difference isthe dimension "L" of the extension element 44. Equation (1) continues toapply. However, the refractive index "n₁ " is taken as unity and therefractive index "n₂ " is determined from the material of the extensionelement 44.

FIG. 4 illustrates another filter 50 with similar input and outputfibers 52 and 54 and lenses 56 and 58. The spectral response function isalso similar. However, the optical path length difference is generatedby a spacer plate 60 made from two concentric elements 62 and 64 thatoccupy different transverse areas of aperture 66. The element 62, whichhas a cylindrical shape, is surrounded by annular-shaped element 64.Each of the elements 62 and 64 is made from a different material--one ofwhich could even be air, but both elements share a common length "L".

Contrast between the respective intensities of the transmitted andnon-transmitted wavelengths can be controlled by adjusting thetransverse areas of the two elements 62 and 64. The respective areas areadjusted by changing the relative sizes of effective radii "r₁ " and "r₂" of the two elements 62 and 64. For example, contrast can be maximizedby relating the respective areas to the intensity profile of theincident collimated beam to balance spectral energies transmitted by thetwo elements 62 and 64. The concentric shapes of the two elements 62 and64 simplify the division of power within beams having radially symmetricpower distributions.

A multi-pass filter 70 illustrated by FIG. 5 is different in severalrespects. Input and output fibers 72 and 74 are located next to eachother on the same side of a single lens 76 but on opposite sides of(i.e., straddle) an optical axis 68. The input fiber 72 emits adiverging beam that is collimated by the single lens 76. A reflectiveoptic 78 retroreflects the collimated beam after passing through aspacer plate 80. Upon its return, the collimated beam is reconverged bythe single lens 76 to a focus at the output fiber 74. To maximizecoupling efficiency, the input and output fibers 72 and 74 can beinclined toward the optical axis 68 in longitudinal alignment with anintersection between the optical axis 68 and the reflective optic 78.

The reflective optic 78 can be formed as a separate mirror or as areflective coating on a remote surface of the spacer plate 80. Elements82 and 84 of the spacer plate 80 are concentric but differ in bothdimensions of length "L₁ " and "L₂ " and in material composition (i.e.,n₁ and n₂). The resulting optical path difference "ΔOPL" accounting forthe reflection can be expressed as follows:

    ΔOPL=2L.sub.1 (n.sub.1 -1)-2L.sub.2 (n.sub.2 -1)     (3)

Despite all of these changes, a spectral response curve similar to FIG.2 is still possible with a contrast governed by radii "r₁ " and "r₂ ".Other shapes and sizes of the spacer elements 82 and 84 could also beused to generate the required optical path length differences.

The single lens 76 could be obviated by shaping the reflective optic asa sphere for reflecting light emanating from the input fiber 72 on areturn course to the output fiber 74. The spacer elements 82 and 84would be reshaped along radial lines to divide portions of the divergingand converging beams.

FIG. 6 depicts a multi-path filter 90 that includes a similararrangement of input and output fibers 92 and 94, together with a singlelens 96 and a reflective element 98 associated with a spacer plate 100for retroreflecting light emitted by the input fiber 92 on a returncourse to the output fiber 94. The spacer plate 100 is distinguished bymultiple annular elements 102, 104, 106, and 108 that vary in lengthfrom "L₁ " to "L_(n) " and that vary in radius from "r₁ " to "r_(n) ".Respective indices "n₁ " to "n_(n) " of the annular elements 102, 104,106, and 108 can be the same or varied. If the same, the optical pathlengths of the corresponding intermediate pathways are varied by thesame combinations of materials (e.g., glass and air) but in differentproportions for exhibiting different effective refractive indices.

The result is a more complex pattern of interference generated by thecombination of additional intermediate beams having respective opticalpath lengths relatively varied by the annular elements 102, 104, 106,and 108 of the spacer plate 100. An exemplary spectral response curve 88of transmitted intensities over a domain of wavelengths is shown in FIG.7. Here, the peak intensities are narrower and more widely spaced thanin the response curve of FIG. 2. Further control over the shape of theresponse curve 88 can be achieved by relatively adjusting the lengths "₁" to "L_(n) " the indices "n₁ " to "n_(n) ", or the radii "r₁ " to"r_(n) " of the annular elements 102, 104, 106, and 108. Variations inthe lengths "L₁ " to "L_(n) " or the indices "n₁ " to "n_(n) " affectthe optical path lengths of the intermediate beams and variations in theradii "r₁ " to "r_(n) " affect their relative power.

The intermediate pathway s th rough the respective optical path lengthdifference generators of all five of the preceding filters illustratedby FIGS. 1-6 have equal physical path lengths but different effectiveindices of refraction. The spacer plates 20 and 60 form equal lengthintermediate pathway s that are distinguished by different materials.The spacer plates 30, 70, and 90 form equal length intermediate pathwaysthat are distinguished by different combinations of materials, includingcombinations that vary only in proportion. The materials themselves candiffer in composition or even physical states, such as glass and air.

A multi-path filter 110 depicted in FIG. 8 is similar in most respectsto the multi-path filter 90 including input and output fibers 112 and114, a single lens 116, a reflective element 118, and a spacer plate120. Also similar to the multi-path filter 90, the spacer plate 120 iscomposed of multiple annular elements 122, 124, 126, and 128 that varyin length from "L₁ " to "L_(n) " and that vary in radius from "r₁ " to"r_(n) ". Respective indices "n₁ " to "n_(n) " of the annular elements122, 124, 126, and 128 can be the same or different.

However, in contrast to all of the preceding embodiments, the relativepositions of the annular elements 122, 124, 126, and 128, together withtheir common reflective element 118, also change the physical pathlengths of the intermediate beams by the differences in their lengths"L₁ " to "L_(n) ". For example, the optical path length differencebetween annular elements 122 and 128 can be expressed as follows:

    ΔOPL=2(L.sub.1 n.sub.1 -L.sub.n n.sub.n)             (4)

To accommodate the different length annular elements 122, 124, 126, and128, the reflective element 118 is stepped. This is most easilyaccomplished by applying a reflective coating to the end faces of theannular elements 122, 124, 126, and 128. If a plane mirror is usedinstead, the physical path lengths of the intermediate beams are equatedsimilar to the embodiment of FIG. 6.

Similar to the embodiments of FIGS. 5 and 6, the single lens 116 can beobviated by reshaping the reflective element 118. However, instead ofshaping the reflective element 118 as a continuous curved surface, theannular steps o f th e reflective element 118 are preferablyindividually curved to provide a similar focusing function. The annularelements 122, 124, 126, and 128 would be tapered along radial lines to aprimary focus.

In any one of the preceding embodiments, the single lenses can bereplaced by diffractive optics or even diffraction pat terns inscribedon the spacers. If inscribed on the reflective surfaces, theintermediate pathways would be shaped to follow the lines of focus.

Like all of the preceding embodiments, the embodiment of FIG. 9 is amulti-path filter 130 having input and output fibers 132 and 134 alongwith a lens 136 or its equivalent. However, the optical path lengthdifference generator is a reflective stack 140 instead of a spacerplate. The reflective stack 140 is made in layers 142 separated bypartially reflective surfaces 144. Each of the layers 142 has a constantlength "L" and a constant index "n", although both the length "L" andthe index "n" could be varied between the layers 142 to provide a morecomplex spectral response.

The reflectivity of the partially reflective surfaces 144 is related tothe number of layers 142 required to establish the desired spectralresponse. For example, if 20 of the layers 142 are required, then eachof the reflective surfaces 144 is made to reflect about five percent ofthe overall spectral energy. The number of layers 142 is analogous tothe number of intermediate pathways through the spacer plates of thepreceding embodiments, and the reflectivity of each of the layers 142 isrelated to the transverse areas of the intermediate pathways. However,the intermediate beams can be separated by less distinct areas of eachpartially reflective surface 144.

In other words, the intermediate beams can occupy virtually the samespace, e.g., the entire aperture "A", upon entry and exit of thereflective stack 140. However, their respective optical path lengths aredistinguished by different physical lengths of travel along an opticalaxis 146 corresponding to a multiple of the length "L". Thus, theequation for optical path length difference between two intermediatebeams from adjacent layers 142 is given by:

    ΔOPL=2L n                                            (5)

The intermediate beams should be separated by an optical path difference"ΔOPL" of at least one wavelength of the propagating light beam.Spacings of at least 10 microns are preferred with average thicknessesexpected in the range of 20 to 30 microns. The reflective surfaces 144can be formed by partially reflective coatings, fully reflectivecoatings (e.g., metallic spots) applied to limited transverse areas, oradjacent material layers having large differences in refractive index.Refractive index differences of at least 1 percent should be used tolimit the number of layers required to reflect substantially all of thepropagating beam, but differences of 10 percent or more are preferredfor constructing practical spectral responses. No more than 100 layersare preferably used.

Additional details regarding the construction and manufacture of similarreflective stacks are contained in my copending U.S. patent applicationSer. No. 08/787,460, filed Jan. 22 1997, and entitled "MultipleReflection Multiplexer and Demultiplexer". This application is herebyincorporated by reference.

Although all of the above embodiments are depicted in bulk optics, thesame embodiments could also be implemented with integrated or hybridoptics. For example, the input and output fibers could be formed aswaveguides on a substrate that also contains one or more focusing andreflecting optics, as well as the optical path length differencegenerator. As a hybrid design, the input and output fibers and the oneor more focusing optics could be formed on a first substrate and theoptical path length difference generator could be formed on a secondsubstrate. Alternatively, the first substrate could also incorporate theoptical path length difference generator, and the reflective elementcould be formed separately as a bulk optic mounted against a wall of thefirst substrate or as a reflective coating on the same wall. Thefocusing optic could also be a curved reflective optic that is formedseparately or as a part of the same integrated device.

Angular tolerances must be tight to assure proper focusing between theinput and output pathways. However, more leeway exists with lateral andlongitudinal dimensions. For example, the lateral position of the spacerplate affects the distribution of light between the intermediate beams,but small variations have only a limited effect on the spectralresponse. Similarly, the length dimension "L" can measure as much as0.25 mm, so small variations in the range of microns are of limitedsignificance.

My invention can also be formed as a part of a single mode optical fiberas shown in FIGS. 10 and 11. A multi-path interference filter 150 isformed between adjacent ends 152 and 154 of the single mode fiber. GRIN(gradient index) fiber rod lenses 156 and 158 and a central spacer 160that functions as an optical path length difference generator are fusionspliced between the adjacent ends 152 and 154.

In one direction of light travel, the GRIN lens 156 collimates lightemitted from fiber end 152. The spacer segment 160 is divided into twoaxial segments 162 and 164 (intermediate pathways) that extend betweenthe GRIN lenses 156 and 158. Each of the segments is made from adifferent material and includes a different refractive index "n₁ " and"n₂ "--at least within the range of wavelengths intended for use. Ajacket 166 can be used to encapsulate the two segments 162 and 164, sotheir respective materials can be in various states including solid andliquid or solid and gas. Photosensitive (e.g., GeO₂ --SiO₂) orelectro-optic (e.g., liquid crystal) materials could also be used.

Similar to the earlier embodiments 10, 30, 50, and 70, the two segments162 and 164 divide the collimated light into two intermediate beamshaving different optical path lengths. The GRIN lens 158 recombines thetwo intermediate beams at a focus on the single mode fiber end 154. Theresulting interference between the combined beams in the fiber end 154produces a spectral response similar to that illustrated in FIG. 2.Additional segments occupying differing amounts of aperture area can beused to provide a more complex response.

Interference filters, including the multi-path interference filters ofthis invention, are subject to manufacturing variations that can affecttheir responses. For example, the actual central wavelengths of thefilters' responses can vary from their intended central wavelengths,sometimes beyond acceptable tolerances. The environmental conditions inwhich the filters are used and the central wavelengths of the signalsintended for filtering are also subject to change. Accordingly, someability to adjust the filters' responses during or after theirmanufacture may be needed to assure their proper functioning.

The tuning of my multi-path interference filters can be performed eitherstatically or dynamically to accomplish a number of different purposes.Static tuning, which permanently changes the filters' spectralresponses, can be carried out during their manufacture to compensate formanufacturing inconsistencies or to achieve improved performance.Dynamic tuning, which produces only a temporary change in the filters'spectral responses, can be carried out during their use to compensatefor changing environmental conditions or signal drift or to performdifferent filtering functions on demand.

Static tuning can be accomplished by using photosensitive materials tovary refractive index as a function of ultraviolet exposure. Forexample, one of the intermediate pathways 162 or 164 can be made with aphotosensitive material such as GeO₂ doped silica, and the other of theintermediate pathways 162 or 164 can be made on a non-photosensitivematerial such as pure silica. By exposing the two pathways 162 and 164to a controlled amount of UV light, the optical path length differencebetween them can be varied to set the central wavelength of the filter'sresponse. The UV exposure can be controlled by active feedback and theexposure stopped when the desired optical characteristics are reached.

In more complicated filter configurations having more than twointermediate pathways, both the amount of doping and the relativeexposure to UV light can be controlled to correct for systematic errorsin the filter's response. The same or different lengths of theintermediate pathways can be exposed to the UV light. Similar resultscan be obtained by stress-induced changes in refractive index. Forexample, a stress-induced change in refractive index can be made bystretching or bending the fiber spacer segment 160 prior to mounting thesegment 160 on a rigid substrate.

Dynamic tuning can be accomplished by using one or more mechanisms suchas thermo-electric effects, electro-optic effects, or stress-opticeffects to temporarily change the optical path lengths of theintermediate pathways. For example, FIG. 12 shows a cross section of analternative spacer 170 having two concentric segments 172 and 174 and ajacket 176 partly surrounded by metal electrodes 178 and 180. The innersegment 172 is made of glass, and the outer segment 174 is made of apolymer that exhibits variations in refractive index with temperature.Varying the temperature of the polymer with the electrodes changes theoptical path length difference between the segments 172 and 174 andthereby alters the spectral response of the filter.

An electro-optical material such as liquid crystal can be substitutedfor the thermosensitive polymer of outer segment 174 and similarlycontrolled by a surrounding electric field to exhibit differentrefractive indices. Mechanically controlled stress can also be used todynamically vary refractive index as well a physical path length. Any ofthese tuning techniques can be applied to the other embodimentsincluding planar, micro-optic, fiber-based or hybrid implementations.

FIGS. 13 and 14 illustrate how to make a GRIN fiber rod lens 182. Alength (approximately 10 cm to 20 cm) of graded index fiber 184 isfusion spliced to a single mode fiber 186. The graded index fiber 184has an index that varies radially according to a Gaussian profile. Thegraded index fiber 184 is mechanically cleaved along line 188 to alength "LG" that is equal to one-quarter of a complete cycle forrefocusing a point source along the optical axis. At the specifiedlength, light from the point source (i.e., the end of the single modefiber) is collimated. Further information about GRIN fiber rod lensescan be found in an article by W. L. Emkey and C. A. Jack entitled"Analysis and Evaluation of Graded-Index Fiber Lenses" from Journal ofLightwave Technology LT-5, 1987, pages 1156-1164. This article is herebyincorporated by reference.

FIG. 15 depicts another embodiment 190 of my multi-path filter, whichaccomplishes the focusing function in a different way. Similar to theembodiments 70 and 90 of FIGS. 5 and 6, a reflective surface 198 couplesadjacent input and output fibers 192 and 194, which straddle an opticalaxis 196. However, the reflective surface 198 is curved (preferablyspherical in bulk implementations) to also provide the focusingfunction. A diverging beam emitted by the input fiber 192 issubstantially retroreflected by the reflective surface 198 on aconverging path to the output fiber 194.

A spacer 200 supporting the reflective surface 198 along the opticalaxis 196 is composed of symmetric (or concentric) optical elements 202and 204, which have different refractive indices. The optical elements202 and 204, which form portions of equal length intermediate pathwaysconnecting the input and output fibers 192 and 194, converge toward afocal point 206 of the reflective surface 198. The different refractiveindices of the optical elements 202 and 204 vary the optical pathlengths of the intermediate pathways for producing a pattern ofinterference that varies the spectral power distribution of therecombined beam reaching the output fiber 194.

A similar focusing function could be performed by a plurality ofindividual reflectives that are offset along an optical axis such asshown in FIG. 8 but oriented to a common focal point. The offset wouldproduce physical path length differences between the intermediatepathways in addition to or as a substitution for differences inrefractive index.

My invention in its various embodiments can be used in a variety offiltering applications over a broad range of wavelengths (e.g., 1200nm-1700 nm) including applications in the fields of communication andsensing technologies. One such application is in conjunction with anoptical amplifier for amplifying specific wavelengths isolated by myfilter.

Although specific embodiments of the invention have been disclosed anddescribed herein, the invention is nonetheless limited only by thefollowing claims.

I claim:
 1. A multi-path interference filter for changing a spectralpower distribution of a beam of light transmitted between input andoutput pathways comprising:an optical path length difference generatoroptically coupling the input and output pathways and having an array ofreflective surfaces that are staggered in a direction of beampropagation between the input and output pathways and are relativelysized to occupy different amounts of transverse area for reflectingcorrespondingly sized portions of the beam through different physicalpath lengths between said input and output pathways; and said reflectivesurfaces being relatively sized in relation to a transverse energydistribution within the beam to enhance interference between thedifferently sized portions of the beam.
 2. The filter of claim 1 inwhich said reflective surfaces are oriented to provide a focusingfunction that couples said optical path length generator to the inputand output pathways.
 3. The filter of claim 1 in which said reflectivesurfaces are concentric for transmitting radially symmetric portions ofthe beam.
 4. The filter of claim 1 further comprising a focusing opticthat couples the input pathway to said optical path length differencegenerator in one direction of the beam propagation and that couples saidoptical path length difference generator to the output pathway in anopposite direction of beam propagation.
 5. The filter of claim 1 inwhich optical path length differences between the differently sized beamportions are further varied by differences between the effectiverefractive indices of the intermediate pathways.